1. Field of the Invention
The invention relates to a receiver for receiving a transmission signal comprising a plurality of (N) frequency multiplexed data modulated carriers, said receiver comprising a frequency multiplex demodulator and a data recovery device having an input for receiving modulation signals of said careers from the frequency multiplex demodulator, and an output for supplying recovered data to further signal processing devices, and having a multiplex data recovery signal path between said input and said output for carrying a multiplex signal having N signal components, each component representing the data of an individual carrier, and having an equalization device and a symbol detection device subsequently arranged in the multiplex signal path.
2. Description of the Related Art
A receiver of this type is known, inter alia from "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", "IEEE Communications Magazine", May 1990, pp. 5 to 14, by John A. C. Bingham.
The known receiver is suitable for receiving audio and/or video information coded and transmitted in a digitized form, hereinafter to be referred to as "data". An example of use is "Digital Terrestrial Television Broadcasting" (DTTB), the future video version of "Digital Audio Broadcasting" (DAB). However, the receiver may alternatively be used in modems for receiving digital information transmitted through cables such as glass fibres, coaxial and multi-core cables for telephony.
In data transmission, the aim is to adapt transmitter(s) and receiver(s) in such a way that the transmission at a given data rate, expressed in bits per second, has a sufficiently low error level, with a favorable exchange between the spectral bandwidth required for transmission and the power of the transmitter. In this respect it should be taken into account that the properties of the transmission channel may vary with time and that these properties may be different for each receiver location.
The afore-mentioned aim is notably important in DTTB and DAB systems. For example, in DTFB, (high-definition) video data may be transmitted within the frequency bands of the present-day analog terrestrial television transmitters. Preferably, the bandwidth required for transmitting the video data should comply with the current channel spacing in these frequency bands, which limits the bandwidth to approximately 7 to 8 MHz. The transmitter power required for the area of coverage should be kept as small as possible so as to limit interferences with analog video signal transmissions. It is also desirable that the technically and/or economically realizable transmitter power covers a maximum possible area.
To limit interferences by and bandwidth coverage of digital video transmissions to a considerable extent, a "single frequency network" (SFN) is considered. The principle of SFN is that various transmitters transmit the same program package at one and the same carrier frequency. These various transmitters are then spread over the desired area of coverage.
A problem in data transmission is that when the received signal is distorted, a larger signal-to-noise ratio at the receiver input is required for regaining the transmitted data substantially without any errors. In the worst case, the distortion is such that the transmitted data cannot be regained or can hardly be regained.
The received signal is notably distorted because the transmitted signal reaches the receiver with different delay times. The first reason is that there are various signal paths between transmitter and receiver in practice, for example, due to reflections of radio signals against objects (such as mountains and buildings). The second reason is an SFN in which delay time differences are produced because transmissions from various sources are received. The delay time differences are dependent on the position of the transmitters with respect to the source and dependent on the position of the receiver with respect to the transmitters.
In the case of delay time differences, the received signal is a sum of undistorted transmitted signals having varying amplitudes, which undistorted signals are shifted in time with respect to each other. The strongest signal in this sum can be considered as the main signal, with leading and trailing echoes. If the transmitted signal is modulated on a carrier with a series of symbols, the modulation of the received signal during a symbol period Ts of an arbitrary symbol will be disturbed by the carrier modulation of previous or possibly subsequent symbols. This form of distortion is referred to as Inter-Symbol Interference (ISI).
The number of previous and subsequent symbols which may disturb a symbol from the received data stream is dependent on the delay time differences in the channel in relation to the symbol frequency Fs. The error probability in the receiver increases with the number of symbols covered by the ISI in the case of a given signal-to-noise ratio of the received signal.
By reducing the symbol frequency Fs, the number of symbols covered by the ISI, as well as the transmission bandwidth decrease. If the quantity of data to be transmitted per unit of time (numbers of bit/s) is to be maintained simultaneously, the factor by which the symbol frequency is decreased will have to be equal to the factor by which the collection of possible symbol values is increased. By increasing the number of possible symbol values, the error probability increases at a given signal-to-noise ratio of the received signal.
In wireless data transmission of, for example video information, delay time differences are a considerable problem. An example is the transmission of data rates of approximately 10 megabits per second through channels having delay time differences increasing to several tens of microseconds. In the case of bivalent symbols ("0" and "1") and a symbol frequency of 10 million per second, the ISI covers several hundred symbols. At a symbol frequency of 10,000 per second the ISI is exclusively limited to the subsequent and/or previous symbol, but there are 1000 possible symbol values (10 bits).
"Multicarrier Modulation" (MCM) is a known technique of adapting transmitters and receivers for transmission via a channel having delay time differences. It provides a flexible exchange between the error probability, required transmission power and bandwidth. MCM is described, for example by John A. C. Bingham in "IEEE Communications Magazine", May 1990, pp. 5 to 14.
FIG. 1 shows an MCM data transmission system. At the transmitter end, a serial-to-parallel converter 1 divides a data stream I into N sub-data streams Is(1) . . . Is(N) having a symbol frequency which is a factor of N lower than the first-mentioned data stream. In a frequency multiplex modulator 2, which comprises a system of modulators m(1) . . . m(N), each sub-data stream Is(1) . . . Is(N) modulates a carrier c(1) . . . c(N) at the frequency f(1) . . . f(N), respectively. The transmitted signal T consists of the addition of all modulated carriers.
At the receiver end, the carriers present in the received signal T' are demodulated in a frequency multiplex demodulator 3 which comprises a system of demodulators r(1) . . . r(N). These demodulators are coupled to the symbol detectors d(1) . . . d(N), respectively, which form pan of a symbol detection device 4. The outputs of the symbol detectors d(1) . . . d(N) supply the sub-data streams Is' (1) . . . Is'(N), respectively. These sub-data streams are combined to a data stream I' by means of a parallel-to-serial converter 5. In the case of transmission without errors, data stream I' will be fully correlated with the transmitted data stream I.
In this technique, it is important that there is no crosstalk between modulation signals of separate carriers, because this may inhibit a flawless reception. Symbols from an arbitrarily chosen sub-data stream Is(x) which modulates carrier c(x) should exclusively contribute to the modulation signal from demodulator r(x) in the receiver.
MCM provides the possibility of choosing the split-up by a factor N of a data stream I to be transmitted into sub-data streams Is(1) . . . Is(N) in such a way that the symbol period in the sub-data streams exceeds the maximum delay time difference in the transmission channel. With a suitably chosen time window, it will then be possible at the receiver end to detect the symbols in the sub-data streams free from ISI.
A practical realization of MCM is also presented by John A. C. Bingham in "IEEE Communications Magazine", May 1990, pp. 5 to 14 and is also known in literature as "Orthogonal Frequency Division Multiplex" (OFDM).
In an OFDM transmitter, shown in FIG. 2, frequency multiplex modulation is effected in a digital signal processor: an N-point Inverse Fast Fourier Transformer (IFFT) 10. N stands for the number of frequency-multiplexed signals. The output of the IFFT 10 supplies the digital baseband signal IF and may be coupled to an output section 11 which is further coupled to the transmission channel. In the case of radio transmission, the output section 11 converts the digital baseband signal into an analog high-frequency signal T. To this end, the output section 11 may comprise, for example, D/A converters, filters, mixer stages and oscillators.
An OFDM receiver has an input section 12 which is coupled to a digital signal processor, being an N-point Fast Fourier Transformer (FFT) 13, which FFT 13 is further coupled to a symbol detection device 4. Input section 12 converts the received analog high-frequency signal into a digital baseband signal IF' and to this end it may comprise, for example, A/D converters, filters, mixer stages, oscillators and tuning circuits. The FFT 13 is a frequency multiplex demodulator.
At the transmitter end, a new group of N symbols from the data stream I is presented to the N inputs of the IFFT 10 in successive periods of time Tb. The IFFT 10 transforms the position-sequential group of N symbols to a time-sequential group of N symbols covering a period of time Tb. Consecutive groups of N symbols associated with consecutive transforms of IFFT 10 constitute the signal IF which is applied to the output section 11.
In the receiver, the output signal IF' of the input section 12 comprises time-sequential groups of N numbers which may be allocated to the respective time-sequential groups of N symbols in signal IF of the transmitter. The received time-sequential groups of N numbers are convened in consecutive periods of time Tb into N position-sequential numbers by the FFT 13. The N position-sequential numbers constitute the respective input signals for the N symbol detectors of the symbol detection device 4, which detectors supply the received N position-sequential symbols.
In essence, a data stream I is transformed to a data stream IF in the transmitter, which transform is performed in groups for N symbols in successive periods Tb. Characteristic of this transform is that each symbol from data stream I, covering time Tb/N and being associated with a group of N symbols, is proportionally allocated to each symbol from the transformed group of N symbols in data stream IF covering time Tb. In other words: the data of a symbol from data stream I, concentrated in a time Tb/N, are spread over a time Th in data stream IF. The aforementioned transform is inversely performed in the receiver. Here, the data of N symbols spread and transmitted over a period of time Tb is converted again into N time-sequential symbols each having a symbol period of Tb/N.
The output signal T of the output section 11 in the OFDM transmitter can be considered as a group of modulated carriers c(1) . . . c(N) having equidistant frequency spacing of fr=1/Tb covering f(1) to f(N). The modulation signal of carrier c(x) at frequency f(x) originates from sub-data stream Is(x).
The output signal T of the output section 11 in the OFDM transmitter may be further considered as systems of N different wave packets g(1,t) . . . g(N,t) which succeed each other with a period Tb. This is shown in FIG. 3A. In the period Tb, the wave packets comprise an undistorted sine-shaped waveform whose frequency for the series of wave packets g(x,t) is equal to f(x). The amplitude and/or phase of a wave packet g(x,T) is dependent on the value of a symbol from the sub-data stream Is(x) at instant T.
The input section 12 in combination with the FFT 13 in the receiver can be considered as a system of N multipliers 15 v(1) . . . v(N), preceding a system of N integrators 16 i(1) . . . i(N), shown in FIG. 3B. The output of multiplier v(x) is coupled to the input of integrator i(x). The received signal appears at the first input of multiplier v(x); the second input of v(x) is controlled by carrier c'(x) at a frequency which is substantially equal to f(x). An undistorted wave packet from the series g(x,t) exclusively yields an output signal at the output of integrator i(x) if the integration period is equal to Tb. In other words: there is no crosstalk between modulation signals of different carriers.
In the case of delay time differences in the transmission channel, the received wave packets are distorted. Within the period of a wave packet, amplitude and/or phase jumps occur due to lagging or leading transitions between consecutive wave packets. Amplitude and/or phase jumps within the afore-mentioned integration period Tb produce output signals at various integrators. A series of wave packets g(x,t), distorted by leading and/or trailing echoes, produces signal values which are unequal to zero at the outputs of integrators other than i(x).
Delay time differences in the channel not only cause crosstalk at the FFT outputs between modulation signals of separate symbols within a sub-data stream, but also crosstalk between those of symbols of different sub-data streams. Thus, there is crosstalk in "time" as well as in "frequency".
A method of inhibiting the inter and intra-sub-data stream crosstalk is to increase the period of time of a wave packet by a "guard" time Tg which is longer than the maximum delay time differences in the transmission channel. The effective symbol period of the sub-data streams is increased to a time Ts=Th+Tg. Within a given chosen time window of length Tb, the received wave packets will be undistorted, i.e. they will have a sine-shaped waveform without amplitude and/or phase jumps. There will be no crosstalk in the output signals of the integrators at an integration period in conformity with this time window. Delay time differences do change the phase and/or amplitude of a wave packet, hence the represented symbol value. This does not cause any problem if the data is stored in the value difference between two consecutive symbols. The reference levels of the symbol detectors can also be adapted with a training cycle in the transmitted signal so that the phase and/or amplitude changes of a wave packet are compensated for.
Increasing the symbol period by a guard time Tg has the drawback that the symbol frequency per sub-data stream decreases by a factor F=(Tg+Tb)/Tb. At an equal bandwidth and an equal number of values per symbol, the transmission capacity also decreases by the factor F. The advantage of insensitivity to delay time differences in the channel is accompanied by the drawback of a less favorable exchange between bandwidth and required signal power.
The afore-mentioned drawback can be alleviated by choosing the factor N of splitting up a data stream I into N sub-data streams to be larger so that Tb will be larger than Tg. In itself, this measure has drawbacks. Firstly, if N is increased, the complexity of the FFT and IFFT circuits in particular will increase. Secondly, if Tb is increased, the requirements imposed on the (instantaneous) frequency accuracy of the oscillators used in the input section 12 will be more stringent with respect to the carriers in the OFDM signal. These requirements may be so stringent that the oscillators cannot be realized, or that they will become very expensive. The frequency accuracy requirements also restrict the rate at which transmitter and receiver move with respect to each other due to the Doppler effect.
Due to the afore-mentioned practical restrictions of increasing the factor N, the use of a guard time may result in a considerable loss of transmission efficiency. This notably applies to transmission with carriers of a very high frequency, such as in the UHF television bands and/or in the case of large delay time differences occurring, for example in an SFN.